1. Field of the Invention
The present invention relates to an acceleration sensor capable of being preferably employed in a webbing retractor for a seat belt adapted to sense the occurrence of an emergency on the basis of the magnitude of acceleration in the horizontal direction and thus to ensure that the occupant restraint webbing is prevented from being drawn out.
2. Related Art
Hitherto, there is known an acceleration sensor of the type described above arranged in such a manner that when a horizontal acceleration with a force exceeding a predetermined magnitude acts, a spherical mass body rises vertically along the inclined slope of a cone and pushes a movable pawl. As a result, the thus-moved pawl prevents rotation of a locking wheel of the webbing retractor so that the webbing is instantaneously prevented from being drawn out from the webbing retractor.
However, in the above-described type of acceleration sensor, the mass body thereof can resonate to the vibration of the vehicle when the vehicle runs over a rough road. As a result, as shown in FIG. 14, a ball 100 can move relative to a vertical axis 104 which passes the center line of the inclined slope 102, causing the ball 100 to be moved even though there is no emergency, and thus locking the webbing for an excessively long time.
This problem is also encountered with other types of acceleration sensors such as tipping type acceleration sensors and swing type acceleration sensors.